The invention relates to telephony equipment having:
an input for receiving useful signals to be transmitted on an outgoing transmission channel, and an output for delivering useful signals received on a return transmission channel, said outgoing and incoming transmission channels being coupled to a telephone line by coupling means,
a service signal detector for detecting service signals transmitted by a telephone exchange on the telephone line and which may be superposed on useful signals.
The invention also relates to a detection method of detecting service signals transmitted to such telephony equipment by a telephone exchange, said service signals which may be superposed on useful signals.
The invention finds interesting applications in the field of telephone transmissions, for example, for implementing services such as the call signal for informing a user who is already on the line that someone else tries to contact him.
Recommendation ETS 300 778-1 by the ETSI (European Telecommunications Standard Institute) notably describes the procedure which is implemented for this purpose between a called party and the telephone exchange on which he depends. This procedure is summarized in FIG. 1. When the telephone exchange 1 receives a call having the called party 2 for its destination, it sends a service signal named CAS (Call Alert Signal) onto the telephone line 3. The called party 2 detects this signal, cuts off the audio channels and sends a DTMF (Dual Tone Multi Frequency) digit xe2x80x9cDxe2x80x9d to the telephone exchange 1. Upon reception of this DTMF xe2x80x9cDxe2x80x9d, the telephone exchange 1 sends the co-ordinates of the calling party (name, telephone number . . . ) to the called party via FSK (Frequency Shift Keying) modulation. Thereafter, the running telephone communication is re-established.
Japanese patent abstract no. 08214055 notably describes a telephone receiving circuit suitable for operating in a CAS detection mode and in an FSK detection mode for implementing such procedure.
The essential problem posed for the detection of service signals, notably CAS signals, is the following. When the service signal is likely to be superposed on other signals, it is necessary to utilize less stringent detection criterions (notably the criterions relating to frequency, duration, signal level . . . ) to ensure the detection thereof whatever the level of these other signals. But when less stringent criterions are used, one runs the risk of false detections, that is to say, detections of service signals in a useful signal (somebody""s voice in the case of a CAS service signal).
It is an object of the invention to propose telephony equipment and a service signal detection method which provide an efficient solution to this problem.
For this purpose, a telephone according to the invention and as defined in the opening paragraph is characterized in that it comprises an echo canceller for cancelling the echo of useful signals coming from said input which come back on said output, the service signal detector being connected between the echo canceller and said output at least when the telephony equipment is on-line.
Similarly, a service signal detection method according to the invention and as described in the opening paragraph is characterized in that it comprises the use of an echo canceller for cancelling the echo of the useful signals coming from said input which come back on said output, and carrying out said detection on the signals produced on the output of the echo canceller.
Thus, the service signal detector neither receives the signals sent by the parties nor the echo of the signal sent by the called party which comes back on the return channel when said coupling means and the telephone line are not properly adapted as regards impedance. The detection criterions used may thus be more stringent which diminishes the risk of false detections and considerably improves the reliability of the system.